The present invention relates to the field of compositions for treatment of a cancer in an animal. In particular, the present invention relates to a method for formulating an aqueous solution of a poorly water-soluble lipophilic camptothecin or camptothecin analog, and to compositions formulated thereby.
Successful formulation of many new drug candidates in a parenteral or oral dosage form is often limited by low solubilities of these drugs in an aqueous solution and/or physical-and chemical instability over a relevant time period. In recent years, progress has been made in developing various solubilization formulation approaches, including salt formation, cosolvent, complexation, mixed micelles/liposomes, emulsions, and micro/nanoparticles. However, for some very poorly water-soluble drugs, solubilization approaches are not sufficient to achieve a desired dosage and a supersaturated solution of drugs stable during parenteral or oral administration may be one of few alternatives.
The inhibition of tumor cell growth by camptothecin analogues is believed to be linked to their action on DNA topoisomerases. Structure-activity studies show that successful inhibition of DNA topoisomerase I by camptothecin analogues requires an intact lactone ring (E-ring, species I in FIG. 1.) functionality. Camptothecin analogues having open lactone ring structures (carboxylate form), while quite soluble in water, are poorly accumulated by cancer cells, exhibit limited activity against the topoisomerase enzyme, and may be more toxic to healthy cells than the lactone form.
Unfortunately, the E-ring intact lipophilic camptothecin analogues have a very poor solubility in water. In the past, poor solubility has prevented the extensive use of highly lipophilic camptothecin analogues in clinical treatment of cancer even though lipophilic camptothecin analogues provide several important advantages over their water-soluble counterparts, such as relatively superior tissue penetration/retention, bioavailability, more consistently optimized dosage and schedule for administration, and less interpatient variability. Indeed, while a number of water soluble camptothecin derivatives including. Camptosar, topotecan, 9-amino-camptothecin, 7-(4-methylpiperazinomethylene)-10,11-methylenedioxy camptothecin, 10,11-methylenedioxy-camptothecin and 10,11-ethylenedioxy-camptothecin have either been on the market, studied preclinically, or used in clinical trials to treat certain types of human cancer, few clinical studies in human patients involving poorly water soluble, highly lipophilic camptothecin analogs [e.g., camptothecin, 10-hydroxy-7-ethyl camptothecin (SN38), and silatecan 7-t-butyldimethylsilyl-10-hydroxycamptothecin (DB-67)] have been conducted. This is largely attributed to the difficulty in developing pharmaceutical formulations that allow the direct administration of the poorly water-soluble, active camptothecin lactone species to human patients with cancer.
Silatecan 7-t-butyldimethylsilyl-10-hydroxycamptothecin (DB-67) is an experimental drug under consideration for clinical testing by the National Cancer Institute for cancer chemotherapy. As shown in FIG. 1, DB-67 is one of a class of A and B ring modified camptothecin analogs. Because of its increased lipophilicity and dual 7-alkylsilyl and 10-hydroxy substitution, DB-67 displays superior binding to cellular and liposomal membranes and enhanced drug stability in the presence of human serum albumin when compared with clinically relevant, more hydrophilic camptothecin analogues. In vitro cytotoxicity assays indicate that DB-67 is of comparable potency to other FDA approved camptothecin analogs (e.g., Camptosar and Hycamtin). A viable formulation for intravenous delivery of DB-67 should maintain a desired dose (1-3 mg/mL) exclusively in the lactone form and of optimal physical and chemical stability.
In accordance with the purposes of the present invention as described herein, a novel method for formulating an aqueous solution of a lipophilic camptothecin or camptothecin analog, and compositions formulated thereby, are provided. In one aspect, the present invention provides a method for preparing a stable supersaturated solution of a lipophilic camptothecin or a camptothecin analog, comprising formulating an alkaline solution of the lipophilic camptothecin or camptothecin analog, and acidifying the alkaline solution in the presence of a solubilizing agent. The alkaline solution may be formulated to have a pH of greater than about 10.0.
The alkaline camptothecin or camptothecin analog may be acidified by diluting into an acidic solution in the presence of a solubilizing agent to achieve a final pH of up to about 6.0. Desirably, the acidic solution may include a buffer suitable for maintaining a pH of said acidic solution at up to about 6.0, for example buffers selected from the group of buffers consisting of, but not limited to, citrate, acetate, lactate, or any mixture thereof.
The solubilizing agent may be selected from a group consisting of a cyclodextrin, a liposome, thermodynamically stable colloidal dispersions (e.g. micelles or microemulsions) containing surface-active agents, an emulsion, or any mixture thereof. Typically, the cyclodextrin solubilizing agent may be a water-soluble xcex2-cyclodextrin derivative, selected from the group consisting of sulfobutyl ether xcex2-cycllodextrin, 2-hydroxypropyl xcex2-cyclodextrin, any other suitably chemically modified xcex2-cyclodextrin, and any mixture thereof. When liposomes are selected as the solubilizing agent, typically they will be formulated as an acidic liposomal suspension having a pH of up to about 6.0. Suitable thermodynamically stable colloidal dispersions may be formulated to have a pH of up to about 6.0, and may comprise a mixture of a surfactant lipid such as Cremophor EL, Vitamin E TPGS, various Pluronics (e.g., polyethylene oxide/polypropylene oxide polymers, Tween 80), and the like. The dispersions may also contain a water miscible co-solvent selected from the group consisting of ethanol, polyethylene glycol (PEG), propylene glycol (PG), glycerol, or any mixture thereof. Dextrose or other suitable excipients may be added to adjust tonicity.
Typically, the lipophilic camptothecin or camptothecin analog may be camptothecin, silatecan 7-t-butyldimethylsilyl-10-hydroxycamptothecin, 10-hydroxy-7-ethyl camptothecin, 9-nitrocamptothecin, silatecan 7-t-butyldimethylsilyl-camptothecin, 7-methylcamptothecin, 7-ethylcamptothecin, 7-propylcamptothecin, 7-butylcamptothecin, or mixtures thereof.
In another aspect, a method for preparing a stable supersaturated solution of a lipophilic camptothecin or a camptothecin-analog is provided, comprising the steps of solubilizing the camptothecin or camptothecin analog of choice in an alkaline solution, and diluting the alkaline solution containing the solubilized camptothecin or camptothecin analog into an acidic solution in the presence of a solubilizing agent, selected from the solubilizing agents as described above. The properties of the alkaline and acidic solutions are as described above. The desired solubilizing agent may be added to the alkaline solution prior to diluting the-alkaline solution into the acidic solution, or may be dissolved into the acidic solution.
The alkaline solution may contain an amount of base (for example, sodium hydroxide) approximately double the concentration of lipophilic camptothecin or camptothecin analog. Typically, the acidic solution will contain an amount of strong acid, such as for example hydrochloric acid, sufficient to neutralize the alkaline solution containing camptothecin or camptothecin analog. Buffers may be included in the acidic solution as described above. Advantageously, the method as described provides a stable, supersaturated composition of lipophilic camptothecin or camptothecin analog which may be lyophilized for long-term storage without significant loss of drug activity upon reconstitution.
In yet another aspect of the present invention, a composition suitable for parenteral or oral administration for the treatment of a cancer in an animal is provided, comprising an aqueous solution containing a therapeutically sufficient amount of a lipophilic camptothecin or camptothecin analog formulated as described above and including the lipophilic camptothecin or camptothecin analog and a solubilizing agent. The lipophilic camptothecin or camptothecin analog may be included in the composition of the present invention in an amount of from about 0.5 to about 3 mg/mL of solution.
The cyclodextrin and/or liposome solubilizing agents may be included in the composition in an amount of from about 10% to about 40% (w/v). In the case where a thermodynamically stable colloidal dispersion (e.g. micelles or microemulsions) is used as the solubilizing agent, an aqueous solution comprising about 12.5% v/v of a polyethoxylated lipid such as Cremophor EL and 12.5% v/v ethanol may be included. In the case where a liposome is used as a solubilizing agent, an acidic liposomal suspension may be used, containing lipids selected from the group consisting of egg phospholipids, cholesterol, dimyristoylphosphatidylcholine, dimyristoylphosphatidylglycerol, or any other suitable lipids or mixtures thereof.
The compositions manufactured by the method of the present invention are advantageously more simple and cost-effective to manufacture, provide a therapeutically suitable dose of poorly water soluble camptothecin or camptothecin analog, are stable during storage, and may be formulated to be free of undesirable organic solvents (e.g., DMSO or N-methyl-2-pyrrolidinone). Advantageously, the compositions may be lyophilized for long-term storage without significant loss of drug activity upon reconstitution for parenteral or oral administration.